1. Field of the Invention
The present invention relates to a method of chemical decontamination for the components of a radioactive material handling facility, such as a nuclear power station, and a system for carrying out the method of removing metal oxides containing radioactive nuclides and adhering to the components of the radioactive material handling facilities from the surfaces of the contaminated components by chemical dissolution.
2. Description of the Related Art
Oxide films containing radioactive nuclides are deposited or formed on the surfaces of components of a nuclear power station in contact with fluids containing radioactive nuclides during operation and subject to radioactive contamination, such as pipes, pieces of equipment and structural members. Consequently, the dose rate around those component members increases and the radiation exposure of workers engaged in work for periodic inspection or dismantlement of a nuclear reactor for decommissioning.
In order to remove the oxide film, a decontaminating solution is supplied the oxide film or a metal forming a contaminated object so as to dissolve them, thereby the oxide film is dissolved in the solution or peeled off into the solution. Aforementioned chemical decontamination method, which dissolves or removes the oxide film chemically, has practically been applied to the decontamination of the components of some nuclear plants and has produced satisfactory results in reducing mediation exposure.
Various chemical decontamination methods intended for the decontamination of stainless steel components of atomic energy plants have been proposed. One of those chemical decontamination methods comprises, in combination, a step of dissolving chromium oxides contained in an oxide film through oxidation using an oxidizing agent, and a step of dissolving ferrous oxides, which are principal components of the oxide film, through reduction a reducing agent.
A chemical decontamination method disclosed in JP B No. Hei 3-10919 employs permanganic acid as an oxidizing agent, and dicarboxylic acid as a reducing agent. The chemical decontaminating method using permanganic acid which has a high oxidizing effect in a low concentration and dicarboxylic acid which can be decomposed into CO2 and H2O produces less secondary wastes as compared with hitherto known chemical decontamination methods and has practically been applied to decontamination work in nuclear power plants.
A decontamination method disclosed in JP A No. Sho 55-135800 uses, in combination, an ozone solution prepared by dissolving ozone in water as an oxidizing agent, and a decontaminating liquid containing an organic acid and a complexing agent. A decontamination method disclosed in JP A No. Hei 9-151798 prepares a foamed decontaminating liquid by blowing ozone gas into a solution containing a foaming agent, and feeds the foamed decontaminating liquid into a contaminated object for decontamination.
When decontaminating contaminated objects by the chemical decontamination method using permanganic acid and dicarboxylic acid as decontaminating agents, the decontaminating agents produce secondary wastes in recovering manganese ion from the permanganic acid solution by means of an ion-exchange resin.
As generally known, ozone is a highly oxidative gas, reacts with water and is decomposed to produce various oxidative active oxygen species. The decontamination method will be a very effective method producing the least amount of secondary wastes attributable to an oxidizing agent if the oxide film can be dissolved in an ozone solution prepared by efficiently dissolving ozone gas in water. However, the ozone concentration of ozone gas produced by a conventional silent discharge ozonizer is low (in general, lower than 1% by volume), and the ozone concentration of ozone solution prepared by supplying the ozone gas in an acid solution is several parts per million or less.
An oxidation process using an ozone solution having such a low ozone concentration, as compared with a conventional chemical decontamination method using permanganic acid, has an inferior oxide film removing ability. To make matters worse, ozone decomposes in water and the ozone concentration of the ozone solution decreases. When the temperature of the ozone solution is high, the half life of dissolved ozone is short and it is possible that ozone disappears in a few minutes. The higher the temperature of the decontaminating liquid for the chemical decontamination method, the higher is the rate of dissolution of the oxide film and the higher is the decontaminating effect. Therefore, the chemical decontamination method must be carried out at temperatures as high as possible to reduce overall time necessary for decontamination work.
Although it is possible to hold ozone gas in foams produced in the decontaminating liquid by a foaming agent thereby holding ozone in the decontaminating liquid for a long time, the foaming agent produces secondary wastes.
The known chemical decontamination method using oxidation and reduction is applied mainly to decontaminating stainless steel components and cannot be applied to decontaminating components made of metallic materials susceptible to the corrosion by chemicals, such as carbon steels.